OMAPDSS: HDMI: fix missing unlock on error in hdmi_dump_regs()

Add the missing unlock on the error handling path in function
hdmi_dump_regs().

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Reviewed-by: Sumit Semwal <sumit.semwal@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>